filescontrol: silence suspicious RCU warning

hulk inclusion
category: bugfix
bugzilla: 27214
CVE: NA
---------------------------

files_fdtable() in files_cgroup_count_fds() should be invoked under
files_struct->file_lock, otherwise a suspicious RCU usage warning
triggers below when CONFIG_PROVE_RCU and CONFIG_LOCKDEP are enabled.

  =============================
  WARNING: suspicious RCU usage
  ...
  -----------------------------
  fs/filescontrol.c:96 suspicious rcu_dereference_check() usage!
  ...
  stack backtrace:
  CPU: 0 PID: 0 Comm: swapper/0 Not tainted
  4.19.36-cph920-32bitc-vmalloc-binder-debugon.aarch64 #6
  Call trace:
   dump_backtrace+0x0/0x198
   show_stack+0x24/0x30
   dump_stack+0xd0/0x11c
   lockdep_rcu_suspicious+0xcc/0x110
   files_cgroup_count_fds+0xc0/0xe0
   dup_fd+0x234/0x448
   copy_process.isra.2.part.3+0x698/0x1490
   _do_fork+0xe8/0x728
   kernel_thread+0x48/0x58
   rest_init+0x34/0x2a0
   start_kernel+0x52c/0x558

Although the 'newf' is newly created and will not be released in
paralle, still silence the warning through adding spin_lock around.

Fixes: 52cc1eccf6de ("cgroups: Resource controller for open files")
Signed-off-by: Nzhangyi (F) <yi.zhang@huawei.com>
Reviewed-by: Nyangerkun <yangerkun@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

fs/file.c

@@ -361,8 +361,12 @@ struct files_struct *dup_fd(struct files_struct *oldf, int *errorp)
 
 	rcu_assign_pointer(newf->fdt, new_fdt);
 #ifdef CONFIG_CGROUP_FILES
-	if (!files_cgroup_alloc_fd(newf, files_cgroup_count_fds(newf)))
+	spin_lock(&newf->file_lock);
+	if (!files_cgroup_alloc_fd(newf, files_cgroup_count_fds(newf))) {
+		spin_unlock(&newf->file_lock);
 		return newf;
+	}
+	spin_unlock(&newf->file_lock);
 
 	/* could not get enough FD resources.  Need to clean up. */
 	new_fds = new_fdt->fd;